Re: [PATCH 7/7] xfs: only run COW extent recovery when there are no live extents

Dave Chinner
2021-12-16T04:41:06Z

On Wed, Dec 15, 2021 at 05:09:54PM -0800, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
>
> As part of multiple customer escalations due to file data corruption
> after copy on write operations, I wrote some fstests that use fsstress
> to hammer on COW to shake things loose. Regrettably, I caught some
> filesystem shutdowns due to incorrect rmap operations with the following
> loop:
>
> mount <filesystem> # (0)
> fsstress <run only readonly ops> & # (1)
> while true; do
> fsstress <run all ops>
> mount -o remount,ro # (2)
> fsstress <run only readonly ops>
> mount -o remount,rw # (3)
> done
>
> When (2) happens, notice that (1) is still running. xfs_remount_ro will
> call xfs_blockgc_stop to walk the inode cache to free all the COW
> extents, but the blockgc mechanism races with (1)'s reader threads to
> take IOLOCKs and loses, which means that it doesn't clean them all out.
> Call such a file (A).
>
> When (3) happens, xfs_remount_rw calls xfs_reflink_recover_cow, which
> walks the ondisk refcount btree and frees any COW extent that it finds.
> This function does not check the inode cache, which means that incore
> COW forks of inode (A) is now inconsistent with the ondisk metadata. If
> one of those former COW extents are allocated and mapped into another
> file (B) and someone triggers a COW to the stale reservation in (A), A's
> dirty data will be written into (B) and once that's done, those blocks
> will be transferred to (A)'s data fork without bumping the refcount.
>
> The results are catastrophic -- file (B) and the refcount btree are now
> corrupt. In the first patch, we fixed the race condition in (2) so that
> (A) will always flush the COW fork. In this second patch, we move the
> _recover_cow call to the initial mount call in (0) for safety.
>
> As mentioned previously, xfs_reflink_recover_cow walks the refcount
> btree looking for COW staging extents, and frees them. This was
> intended to be run at mount time (when we know there are no live inodes)
> to clean up any leftover staging events that may have been left behind
> during an unclean shutdown. As a time "optimization" for readonly
> mounts, we deferred this to the ro->rw transition, not realizing that
> any failure to clean all COW forks during a rw->ro transition would
> result in catastrophic corruption.
>
> Therefore, remove this optimization and only run the recovery routine
> when we're guaranteed not to have any COW staging extents anywhere,
> which means we always run this at mount time. While we're at it, move
> the callsite to xfs_log_mount_finish because any refcount btree
> expansion (however unlikely given that we're removing records from the
> right side of the index) must be fed by a per-AG reservation, which
> doesn't exist in its current location.
>
> Fixes: 174edb0e46e5 ("xfs: store in-progress CoW allocations in the refcount btree")
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>

Looks good now.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
--
Dave Chinner
david@fromorbit.com